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The utilization of cellulose nano fibers (NCell) as reinforcement to cement composites may account for enhancing particle packing and decreasing the crack growth at the nano level. Besides that, their high specific surface area can improve the bond between the cement hydration products. nano silica particles can also influence the performance of fiber reinforced cement composites through increasing the calcium-silicate-hydrate gel content, as well as enhancing the fiber to matrix interface cohesion. The aim of this investigation is to evaluate the effects of different contents of nano silica particles (0, 1, 1.5, and 2% by weight of cement) on the mechanical properties, and microstructure of cement composites reinforced with various contents of nano cellulose fibers (0, 0.35, 0.55, and 0.75% by weight of cement). The results out of studying such nano systems revealed that the addition of the NCell particles separately, helped in improving the tensile strength by about twice the value of the control mix when appropriate content of NCell was incorporated (0.35%). Moreover, the microstructural analyses demonstrated the effectiveness of nano silica particles to modify the transition zone between the nano cellulose fiber and the cementitious matrix and subsequently allowing greater reinforcement efficiency especially when external compression or tensile forces were transferred to the composite.